MOS circuit for lowering forward voltage of diodes

ABSTRACT

A circuit for lowering the effective voltage drop of a semi-conductor diode. The circuit includes a first Metal Oxide Semi-conductor (MOS) device connected in parallel with the diode. A bias voltage which is close to but lower than the threshold voltage is applied to the MOS device. When a forward voltage is applied to the diode, this voltage is added to the source to drain voltage of the MOS device which turns it on. The MOS device then bypasses the diode to effectively overcome the inherent forward voltage drop of the diode. The circuit is advantageously applied to a rectifier circuit to reduce input voltage requirements and improve the efficiency of the rectifier.

FIELD OF THE INVENTION

[0001] This invention relates to circuits having semi-conductor diodesand more particularly to a system and method of lowering the effectiveforward voltage drop of semi-conductor diodes.

BACKGROUND

[0002] Semi-conductor diodes are used extensively in half wave and fullwave rectifier circuits. It is well known that semi-conductor diodesconduct current in a forward direction but substantially no currentflows when the diode is reverse bias. Further, in the forward directioncurrent only flows after the applied voltage exceeds a predeterminedvoltage and this voltage is known as the forward voltage drop. The valueof the forward voltage drop is dependent on the material from which thesemi-conductor diode is fabricated. The forward voltage drop of asilicon diode, for example, is typically 0.7 volts. It is, of course,known to manufacture diodes by other techniques which may result in alower forward voltage drop. For example, Schottky diodes have a lowforward voltage drop but the process necessary to fabricate Schottkydiodes is not always compatible with the process used in manufacturingthe integrated circuit incorporating the semiconductor diode.

[0003] There are several applications wherein the input voltage to thecircuit is limited and the forward voltage drop of a standard diode mayadversely affect the efficiency of the circuit such as, for example, arectifier circuit.

[0004] In the particular application for which a low forward voltagedrop is advantageous is an RF-tag circuit. RF tag circuits are used tocommunicate various forms of information from and to remote locations.These can be active RF tags which automatically transmit information orsemi-passive RF tags which are activated or interrogated by anotherdevice.

[0005] The present invention applies in particular to systems whereinthe RF tag is interrogated and in response returns information such aslocation, temperature, humidity, pressure etc. In such systems therectifier in the input circuit rectifies incoming data from an externalsource and extracts from the incoming data the supply voltage (vdd andvss) for the integrated circuit in the RF tag unit.

[0006] Obviously, the forward voltage drop can affect adversely is theefficiency of the rectifier circuit. Therefore, there is a need for asystem and method to effectively lower the forward voltage drop ofdiodes used in low voltage environment.

SUMMARY OF THE INVENTION

[0007] The present invention provides a simple circuit that can reduceforward voltage drop of diodes and provide thereby increased efficiencyand output voltage of a rectifier circuit.

[0008] A current reference controls secondary effects of the circuit.The circuit of the present invention is of particular value inintegrated circuit fabrication where standard application processes areemployed and Schottky diodes are not available.

[0009] The invention is also well suited for low voltage and low powerapplications where the forward voltage of a diode limits the dynamicrange.

[0010] Therefore, in accordance with a first aspect of the presentinvention there is provided a circuit for lowering the effective forwardvoltage drop-of a semi-conductor diode comprising: a first metal oxidesemi-conductor (MOS) device connected in parallel with thesemi-conductor diode, the MOS device having a set threshold voltage; anda second MOS a device for establishing a bias voltage for the first MOSdevice, the bias voltage being less than the threshold voltage by asmall value.

[0011] In accordance with a second aspect of the present invention thereis provided a full wave rectifier circuit comprising first and secondcomplementary diodes, each of the diodes having an associated circuitfor lowering its effective forward voltage drop, the associated circuitcomprising: a first metal oxide semi-conductor (MOS) device connected inparallel with each of the semi-conductor diodes, the MOS device having aset threshold voltage; and a second MOS device for establishing a biasvoltage for the first MOS device, the bias voltage being less than thethreshold voltage by a small value.

[0012] In accordance with a further aspect of the present inventionthere is provided a method of reducing the effective voltage drop of asemi-conductor diode comprising: connecting a first metal oxidesemi-conductor (MOS) device in parallel with the diode, the first MOSdevice having a predetermined threshold voltage; and generating a biasvoltage for the first MOS device, the bias voltage being less than thethreshold voltage; whereby input voltage applied to the diode is addedto the bias voltage to turn on the MOS device, thereby bypassing thediode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will now be described in greater detail withreference to the attached drawings wherein:

[0014]FIG. 1 is a circuit diagram and a prior art rectifier circuit;

[0015]FIG. 2 is a circuit diagram of a single diode with 6 bypasscircuitry;

[0016]FIG. 3 is a circuit diagram of a full wave rectifier circuit usingthe circuitry of the present invention; and

[0017]FIG. 4 is a circuit diagram of a RF tag circuit using the forwardvoltage drop lowering circuit of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018]FIG. 1 is a circuit diagram of a known full wave rectifier. Apositive voltage at terminal 12 causes current to flow through diode D1after the forward voltage drop has been exceeded. Similarly, a positivevoltage at terminal 14 causes current to flow through diode D2 againafter the forward voltage drop has been exceeded. As indicatedpreviously this forward voltage drop is dependent on the material usedto manufacture the semi-conductor diodes D1, D2 but for a silicon baseddiode the value is 0.7 volts.

[0019]FIG. 2 illustrates the basic implementation of the presentinvention. In this implementation MOS device M1 having source, drain andgate terminals is connected as shown. The source and drain connectionsare on either side of diode D1. It is, of course, known that in MOSdevices the flow of source to drain current is controlled by the gatevoltage. In the implementation of FIG. 2 the gate voltage is biasedclose to but less than the inherent threshold voltage Vt of MOS deviceMl. As is known the threshold voltage is the gate to source voltagelevel at which current flows through the source to drain connections.This bias voltage is derived from MOS device M2 in series with areference current source is as shown.

[0020] When an increasing input voltage is applied to diode D1 icy thevalue of this voltage is added to the bias voltage bias which veryquickly overcomes the threshold voltage and MOS device Ml is turned on.The MOS device Ml now provides a low resistance path in parallel withdiode Dl thereby overcoming the forward voltage drop of the diode.

[0021]FIG. 3 illustrates a full wave rectifier circuit comparable to thecircuit of FIG. 1 but now incorporating the bypass circuitry of thepresent invention. In this case complementary metal oxide semi-conductor(CMOS) devices are used in each arm of the rectifier circuit. In theembodiment shown in FIG. 3, MOS devices Ml and M2 provide a bypasscircuit for diode D1 when terminal 12 is provided with a positionvoltage. Similarly MOS devices M3 and M4 provide the bypass circuitryfor diode D2 where a position voltage appears at terminal 14. MOSdevices Ml and M2 and M3 and M4 respectively operate as current mirrorsso that when diodes D1 or D2 are reverse biased the leakage current ofeach diode is set by the reference current and the current mirror ratio.

[0022] For a low power application, as in an RF-tag application, thiscurrent can be practically negligible. In the circuit of FIG. 3 thereverse leakage current does not load the external resource circuit astheir current is only the difference or mismatch in leakage currentsbetween the two complementary circuits.

[0023] In an RF tag application the output voltage using the circuitryof the present invention is improved from 1.2 to 1.8 volts.Additionally, there is a much higher loading capacity due to the lowerlosses in the rectifier.

[0024] The diodes D1 and D2 are required in the modified circuit for theinitial time period before vdd is high enough to turn on the currentreference in the integrated circuit.

[0025]FIG. 4 is a circuit diagram of an IC implementing an RF tag. Asshown on the input side of the circuit the full wave rectifierincorporates the diode bypass circuitry of the present invention.

[0026] While a particular embodiment of this invention has beendisclosed and illustrated it will be apparent to one skilled in the artthat changes can be made to the circuit without substantially alteringthe basic concept of the invention.

[0027] Such changes, however, will be encompassed in the full scope ofthe invention as defined by the appended claims.

1. A circuit for lowering the effective forward voltage drop of asemi-conductor diode comprising: a first metal oxide semi-conductor(MOS) device connected in parallel with said semi-conductor diode, saidMOS device having a set threshold voltage; and a second MOS device forestablishing a bias voltage for said first MOS device, said bias voltagebeing less than said threshold voltage by a small value.
 2. The circuitas defined in claim 1 wherein said MOS devices each have a source, adrain and a gate connections, said source and drain of said first MOSdevice being connected in parallel with said diode, and said gateconnected to said second MOS device to derive a bias voltage.
 3. Thecircuit as defined in claim 2 wherein said second MOS device isconnected to a reference current source.
 4. A full wave rectifiercircuit comprising first and second complementary diodes, each of saiddiodes having an associated circuit for lowering its effective forwardvoltage drop, said associated circuit comprising: a first metal oxidesemi-conductor (MOS) device connected in parallel with each of saidsemi-conductor diode, said MOS device having a set threshold voltage;and a second MOS device for establishing a bias voltage for said firstMOS device, said bias voltage being less than said threshold voltage bya small value.
 5. A full wave rectifier circuit as defined in claim 4wherein complementary metal oxide semi-conductor (CMOS) devices are usedin association with said complementary diodes.
 6. A method of reducingthe effective voltage drop of a semi-conductor diode comprising:connecting a first metal oxide semi-conductor (MOS) device in parallelwith said diode, said first MOS device having a pre-determined thresholdvoltage; and generating a bias voltage for said first MOS device, saidbias voltage being less than said threshold voltage; whereby inputvoltage applied to said diode is added to said bias voltage to turn onsaid MOS device, thereby bypassing said diode.
 7. The method as definedin claim 6 wherein said diode is in a half wave rectifier circuit. 8.The method as defined in claim 6 wherein a pair of diodes are used in afull wave rectifier circuit.